You will likely want to use a "TreeView" control of some sort -- the one built in to ASP.NET is ok and would work for this purpose, but there are more sophisticated ones available (at a cost).

At any rate, you'd want to get the base path and then iterate through all sub directories (use recursion to go deeper) and populate the treeview.  A sample snippet as follows (with no recursion, but should give the general concept):

string rootPath = Server.MapPath("~/SomePath");
System.IO.DirectoryInfo dir = new System.IO.DirectoryInfo(rootPath);
foreach (System.IO.DirectoryInfo subDir in dir.GetDirectories())
{
// add tree item
}

That's true, I forgot about that one... we have that one in use with our skin, but spaced out that they could actually try to use that instead of the built in one and as an alternative to the ones that cost money.  Nice catch.

I haven't personally tried the DNN treeview aside from the skin, so I don't really have any comments on it.